Imaging systems such as printers, fax machines, and copiers are virtually omnipresent, and can be found in homes and offices worldwide. The development of such systems has facilitated improvements in communication that have in turn fostered a sea of change in the way people live and work. Telecommuting, paperless offices, and intra-office networks represent but a few examples of the advancements that have been made possible by modern imaging systems.
Since these systems have become crucial to everyday existence, their reliability and smooth operation is paramount. It is therefore vitally important to design imaging systems so that downtime and work interruptions are minimized. This can be a daunting challenge, given the relative complexity of systems in which sheet material must be infed, moved through the imaging process, and outfed in a matter of seconds.
One common and recurring problem in imaging systems is document misfeed, which can result in sheet material such as paper getting lodged in the transport mechanism. This condition, often referred to as a "jam", is a source of frustration for system users.
One cause of such jams is misalignment between elements of the sheet feed drive mechanism. As shown in FIG. 1, a typical drive mechanism D includes a driven friction tire arrangement F mounted on an upper shaft U. Support rollers S are mounted on a lower shaft L in proximity with the friction tires F. Transport force is imparted to the sheet material M as it passes between the friction tires F and the support rollers S.
As long as the upper shaft U and lower shaft L remain relatively parallel as shown in FIG. 1, the friction tires F contact the support rollers S with approximately the same force, and thus produce approximately the same transport force with little or no skew.
Unfortunately, the shafts are seldom parallel in practice. When the upper shaft U and lower shaft L are out of parallel alignment, as shown in FIG. 2, the friction tires F do not contact the support rollers S squarely. Consequently, the friction tires F and support rollers S will produce uneven transport forces, and therefore skew the sheet being transported.
In an attempt to mitigate this problem, gear wheel assemblies can be constructed as shown in FIGS. 3 and 4. The gear wheel assembly G includes an annular cylindrical gear wheel W. The gear wheel W has an internal gear tooth profile T on each end. A pair of toothed pinions P are mounted on a shaft A. The teeth of the pinions P are square, and are intended to mesh with the tooth profiles T to drive the gear wheel W. The pinions P have an external diameter substantially smaller than the internal diameter of the gear wheel W.
The pinions P do not allow a high degree of tolerance for misalignment. When the upper shaft U and lower shaft L are significantly out of parallel alignment, the individual pinions tend to "jump" a tooth, causing one pinion to be one or more teeth "ahead of" or "behind" the other. This situation places the gear wheel W in an asymmetrical position, resulting in a skewed sheet.
It can thus be seen that the need exists for a drive mechanism that will increase the reliability of sheet feed arrangements by minimizing skewing.